Within minutes after IP injection of labeled carcinogens into rats, there is a noticeable uptake by cells, and radioactivity associated with cytoplasmic proteins and nuclei. Over 90 percent of the total nuclear binding is associated with the isolated chromatin. Similar results are observed when mouse embryo cell cultures are exposed to the radioactive hydrocarbons. Since chemical carcinogens are known to cause chromosomal aberrations, bind to DNA, alter DNA dependent RNA synthesis, and activate latent viral genes incorporated into host cell chromosomes, it is proposed that carcinogens mediate all or part of the action on cells at the level of chromatin. It is planned to continue studies on the nature of the interaction of carcinogens with chromatin in vivo and in vitro, the nature of the compound bound in vivo after exposure to whole cells, as well as the component on chromatin to which the carcinogen is bound. Emphasis will be placed on the possible similarities that chemical carcinogens have with steroid hormones with respect to cytoplasmic receptors, migration to the nucleus and binding to chromatin, and the resulting alteration in transcription (gene expression).